Mr Ai, the internationally famous artist and outspoken government critic, was last seen on Sunday in police custody after he was barred from boarding a flight at a Beijing airport. His disappearance comes as the security services carry out a crackdown on lawyers, writers and activists following calls for protests similar to those in the Middle East and North Africa.

Lu Qing, Mr Ai's wife, said the detention seemed different from his previous run-ins with the authorities: "This time it's extremely serious. They searched his studio and took discs and hard drives and all kinds of stuff, but the police haven't told us where he is or what they're after. There's no information about him."

Chinese police also called more people in for questioning yesterday [Tues] as they expanded their investigation. Alison Klayman, an American filmmaker who has been working on a documentary about the artist, said police appeared to be working their way down a list of Chinese citizens and foreigners associated with Mr Ai.

Britain, France, Germany, the US and the European Union have now all issued calls for an end to a wave of arbitrary detentions in China and the release Mr Ai who was taken away by police as he tried to board a flight to Hong Kong for a business trip.

The EU delegation in China released a statement saying it was "concerned" by the increasing use of detentions against activists, specifically noting the case of Mr Ai. "We call on the Chinese authorities to refrain from using arbitrary detention under any circumstances," the statement added.

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William Hague, the foreign secretary, has also demanded that China explain why it had detained the flamboyant artist-activist whose Sunflower Seeds has been exhibiting at the Tate Modern's turbine hall this year. "I call on the Chinese government to urgently clarify Ai's situation and wellbeing, and hope he will be released immediately," said Mr Hague in a statement, adding that human rights and the rule of law were "essential prerequisites" to China's long-term prosperity and stability.

"The response from the West has been toothless. It is time now that China was referred to the UN's Human Rights council, because these disappearances are such a departure from China commitment to UN mechanisms," said Nicholas Bequelin of Human Rights Watch in Hong Kong.

Mr Ai, a trenchant critic of the failing of China's one-party state has in the past enjoyed a measure protection in the past because of his position as the son of the late Ai Qing, one of China's greatest modern poets, but this luck now appears to be at an end.